I have a 105 triple chainset (hollowtechII) and the left hand side has developed a crack. Now I can't find any replacement 105 LH cranks but there are some Deore ones on the ebays that aren't too expensive.

So will they fit? I know since it's Shimano nothing is compatible with anything else, you're supposed to just go out and buy a whole new bicycle, but I'm hoping…

I know since it's Shimano nothing is compatible with anything else. ??? Not in my expereince. Shimano's different ranges tend to be very compatible with each other. But as @Alex says the Deore is a Mountain range and you need one of the Road ranges. Check out Tiagra or Sora (Ultegra will also be compatible but if bought new, will be more expensive than 105)
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PeteHFeb 14 '13 at 21:09

4 Answers
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Mountain bikes have wider chain stays than road bikes so the cranks need to be further apart to prevent them (or the cyclist's heel) from banging into the frame. The distance between the pedal attachment points on the crank arms is referred to as the Q factor. Typical Q factor for road cranks is ~150mm; for mountain cranks it is ~175mm. This means a mountain crank is likely to be 10-15mm further from the frame than a road crank.

Small differences in the crank symmetry aren't that noticeable, but I suspect that 15mm might be. Also worth noting that many people believe that narrower cranks are more comfortable and efficient, which is why manufacturers produce their road cranks with a lower Q factor.

If you want to replace your cranks on the cheep, I would try to find a second hard pair compatible with your existing chain rings and bottom bracket. Do swap the chain rings, even if the replacement crank has rings, because differences in the wear between rings and chain can accelerate wear and cause chain slippage.

As mentioned by Alex, Hollowtech parts are compatible. If all you're replacing is the left-hand crank arm, the only thing that you need to worry about is that it is the same length as your existing crank arm. Whether it's for mountain or road bikes, crank arms come in a variety of different lengths.

The weight will likely be off if you get any type of crank other than what's already on there. Cheap cranks tend to be heavier on average than more expensive cranks. But you're probably not going to notice the difference. Except maybe when you take your feet off the pedals with the heavy one in the back position and gravity pulls it downward.

Other than that, a mountain crank will probably look funny and that's about it.